1. Field of the Invention
The present invention relates to a radio receiver.
2. Description of the Related Art
The conventional technique disclosed, for example, U.S. Pat. No. 5,020,147 xe2x80x9cFM/AM broadcast signal converterxe2x80x9d correspondent to the Japanese Patent Application Laid-open No. 1-273432) mentions that an FM broadcast receiver can be integrated into one chip to adopt the direct conversion system instead of the superheterodyne system.
FIG. 1 illustrates one example of such a receiver, in which the FM intermediate frequency is 150 kHz, the part surrounded by a chain line is integrated into one chip as an IC 10, and the components or circuits placed outside the chain line are connected to the IC 10 from outside, thus forming a receiver.
And, of the outside components or the outside circuits, the symbol 41 indicates an antenna, the symbol 42 a pre-selector (band pass filter) of which pass band is the FM band, the symbol 43 indicate a resonant circuit for a local oscillator, which is formed of a coil and variable capacitor, and the symbol 44 indicates a speaker.
Upon reception of an FM broadcast signal, the signal received by the antenna 41 is supplied to mixing circuits 12I and 12Q through the signal line of the pre-selector 42 and a radio frequency amplifier 11. Here, suppose that the objective received signal SRX is given by:
SRX=A sin xcfx89RXt, 
xcfx89RX=2xcfx80fRX. 
And, in the subsequent signal processing, only the relative amplitude and phase of the signals are involved; and in the description of the foregoing equation and the following, the initial phase of the signals will be omitted.
And, assuming that the local oscillation frequency is given by:
fLO=fRXxe2x88x92fIF 
fIF: FM intermediate frequency (=150 kHz), a local oscillation circuit 21 generates a local oscillation signal SLO having double the frequency of the original local oscillation frequency fLO.
And, the signal SLO is supplied to frequency dividing circuits 22I, 22Q and the frequency is divided into half. As shown in FIG. 5, for example, the frequency dividing circuits 22I, 22Q generate signals SLI, SLQ, respectively, that reverse at a zero-crossing point in the rising phase and at a zero-crossing point in the falling phase. That is, the signal SLO is divided into the signals SLI, SLQ having the frequency fLO and the phase difference of 90xc2x0 between the two, expressed as:       SLI    =          B      ⁢              xe2x80x83            ⁢      cos      ⁢              xe2x80x83            ⁢      ω      ⁢              xe2x80x83            ⁢      LO            SLQ    =          B      ⁢              xe2x80x83            ⁢      sin      ⁢              xe2x80x83            ⁢      ω      ⁢              xe2x80x83            ⁢      LO                  ω      ⁢              xe2x80x83            ⁢      LO        =          2      ⁢      π      ⁢              xe2x80x83            ⁢              fLO        .            
Incidentally, in the description of the foregoing equation and the following, only the fundamental components (SLI and SLQ) will be expressed as to the SLI SLQ SLQ for simplification, and the harmonics of them will be omitted.
These signals SLI, SLQ are supplied to the mixing circuits 12I, 12Q, respectively, as the local oscillation signals.
Therefore, output signals SII, SIQ of the mixing circuits 12I, 12Q will be given by the following.                               SII          ⁢                      xe2x80x83                    =                      xe2x80x83                    ⁢                      SRX            ·            SLI                                                        =                      A            ⁢                          xe2x80x83                        ⁢            sin            ⁢                          xe2x80x83                        ⁢            ω            ⁢                          xe2x80x83                        ⁢            RXt            xc3x97            B            ⁢                          xe2x80x83                        ⁢            cos            ⁢                          xe2x80x83                        ⁢            ω            ⁢                          xe2x80x83                        ⁢            LO                                                                    =                          xe2x80x83                                ⁢          α          ⁢                      xe2x80x83                    ⁢                      {                                          sin                ⁢                                  xe2x80x83                                ⁢                                  (                                                            ω                      ⁢                                              xe2x80x83                                            ⁢                      RX                                        ⁢                                          xe2x80x83                                        +                                          xe2x80x83                                        ⁢                                          ω                      ⁢                                              xe2x80x83                                            ⁢                      LO                                                        )                                ⁢                                  xe2x80x83                                ⁢                t                            ⁢                              xe2x80x83                            +                              xe2x80x83                            ⁢                              sin                ⁢                                  xe2x80x83                                ⁢                                  (                                                            ω                      ⁢                                              xe2x80x83                                            ⁢                      RX                                        ⁢                                          xe2x80x83                                        -                                          xe2x80x83                                        ⁢                                          ω                      ⁢                                              xe2x80x83                                            ⁢                      LO                                                        )                                ⁢                                  xe2x80x83                                ⁢                t                                      }                                                            SIQ          ⁢                      xe2x80x83                    =                      xe2x80x83                    ⁢                      SRX            ·            SLQ                                                        =                      A            ⁢                          xe2x80x83                        ⁢            sin            ⁢                          xe2x80x83                        ⁢            ω            ⁢                          xe2x80x83                        ⁢            RXt            xc3x97            B            ⁢                          xe2x80x83                        ⁢            sin            ⁢                          xe2x80x83                        ⁢            ω            ⁢                          xe2x80x83                        ⁢            LO                                                                    =                          xe2x80x83                                ⁢          α          ⁢                      xe2x80x83                    ⁢                      {                                                            -                  cos                                ⁢                                  xe2x80x83                                ⁢                                  (                                                            ω                      ⁢                                              xe2x80x83                                            ⁢                      RX                                        ⁢                                          xe2x80x83                                        +                                          xe2x80x83                                        ⁢                                          ω                      ⁢                                              xe2x80x83                                            ⁢                      LO                                                        )                                ⁢                                  xe2x80x83                                ⁢                t                            ⁢                              xe2x80x83                            +                              xe2x80x83                            ⁢                              cos                ⁢                                  xe2x80x83                                ⁢                                  (                                                            ω                      ⁢                                              xe2x80x83                                            ⁢                      RX                                        ⁢                                          xe2x80x83                                        -                                          xe2x80x83                                        ⁢                                          ω                      ⁢                                              xe2x80x83                                            ⁢                      LO                                                        )                                ⁢                                  xe2x80x83                                ⁢                t                                      }                                    α    ⁢          xe2x80x83        =          xe2x80x83        ⁢          A      ·              B        /        2            
And, as described later, the signal components of the angular frequency (xcfx89RX+xcfx89LO) are removed from these signals SLI, SLQ, and the signal components of the angular frequency (xcfx89RXxe2x88x92xcfx89LO) are used as the intermediate frequency signal; and for simplicity, ignoring the signal components of the angular frequency (xcfx89RX+xcfx89LO) in the foregoing equation will produce the following.       SII    =          α      ⁢              xe2x80x83            ⁢      sin      ⁢              xe2x80x83            ⁢              (                              ω            ⁢                          xe2x80x83                        ⁢            RX                    -                      ω            ⁢                          xe2x80x83                        ⁢            LO                          )            ⁢      t                  SIQ      =              α        ⁢                  xe2x80x83                ⁢        cos        ⁢                  xe2x80x83                ⁢                  (                                    ω              ⁢                              xe2x80x83                            ⁢              RX                        -                          ω              ⁢                              xe2x80x83                            ⁢              LO                                )                ⁢        t              ⁢          
        ⁢    And    ,          here      ⁢              xe2x80x83            ⁢      the      ⁢              xe2x80x83            ⁢      image      ⁢              xe2x80x83            ⁢      signal      ⁢              xe2x80x83            ⁢      Sim      ⁢              xe2x80x83            ⁢      is      ⁢              xe2x80x83            ⁢      given      ⁢              xe2x80x83            ⁢      by            Sim    =          C      ⁢              xe2x80x83            ⁢      sin      ⁢              xe2x80x83            ⁢      ω      ⁢              xe2x80x83            ⁢      imt                  ω      ⁢              xe2x80x83            ⁢      im        =                  ω        ⁢                  xe2x80x83                ⁢        LO            +              ω        ⁢                  xe2x80x83                ⁢        IF                                ω        ⁢                  xe2x80x83                ⁢        IF            =              2        ⁢        π        ⁢                  xe2x80x83                ⁢        fIF              ,  
and if the received signal SRX from the pre-selector 42 contains the image signal Sim, the signals SII, SIQ in this case are expressed by the following equations.       SII    =                  α        ⁢                  xe2x80x83                ⁢        sin        ⁢                  xe2x80x83                ⁢                  (                                    ω              ⁢                              xe2x80x83                            ⁢              RX                        -                          ω              ⁢                              xe2x80x83                            ⁢              LO                                )                ⁢        t            +              β        ⁢                  xe2x80x83                ⁢        sin        ⁢                  xe2x80x83                ⁢                  (                                    ω              ⁢                              xe2x80x83                            ⁢              im                        -                          ω              ⁢                              xe2x80x83                            ⁢              LO                                )                          SIQ    =                  α        ⁢                  xe2x80x83                ⁢        cos        ⁢                  xe2x80x83                ⁢                  (                                    ω              ⁢                              xe2x80x83                            ⁢              RX                        -                          ω              ⁢                              xe2x80x83                            ⁢              LO                                )                ⁢        t            +              β        ⁢                  xe2x80x83                ⁢        cos        ⁢                  xe2x80x83                ⁢                  (                                    ω              ⁢                              xe2x80x83                            ⁢              im                        -                          ω              ⁢                              xe2x80x83                            ⁢              LO                                )                ⁢        t                  b    =          A      ·              C        /        2            
Further, the following relation is given in this case:
xcfx89RX less than xcfx89LO less than xcfx89im, 
and therefore, the foregoing equations are expressed as follows.                               SII          =                                    αsin              ⁢                              xe2x80x83                            ⁢                              (                                                      ω                    ⁢                                          xe2x80x83                                        ⁢                    RX                                    -                                      ω                    ⁢                                          xe2x80x83                                        ⁢                    LO                                                  )                            ⁢              t                        +                          β              ⁢                              xe2x80x83                            ⁢              sin              ⁢                              xe2x80x83                            ⁢                              (                                                      ω                    ⁢                                          xe2x80x83                                        ⁢                    im                                    -                                      ω                    ⁢                                          xe2x80x83                                        ⁢                    LO                                                  )                                                                                  =                                                    -                α                            ⁢                              xe2x80x83                            ⁢              sin              ⁢                              xe2x80x83                            ⁢                              (                                                      ω                    ⁢                                          xe2x80x83                                        ⁢                    LO                                    -                                      ω                    ⁢                                          xe2x80x83                                        ⁢                    RX                                                  )                            ⁢              t                        +                          β              ⁢                              xe2x80x83                            ⁢              sin              ⁢                              xe2x80x83                            ⁢                              (                                                      ω                    ⁢                                          xe2x80x83                                        ⁢                    im                                    -                                      ω                    ⁢                                          xe2x80x83                                        ⁢                    LO                                                  )                                                                                      SIQ          =                                    α              ⁢                              xe2x80x83                            ⁢              cos              ⁢                              xe2x80x83                            ⁢                              (                                                      ω                    ⁢                                          xe2x80x83                                        ⁢                    RX                                    -                                      ω                    ⁢                                          xe2x80x83                                        ⁢                    LO                                                  )                            ⁢              t                        +                          β              ⁢                              xe2x80x83                            ⁢              cos              ⁢                              xe2x80x83                            ⁢                              (                                                      ω                    ⁢                                          xe2x80x83                                        ⁢                    im                                    -                                      ω                    ⁢                                          xe2x80x83                                        ⁢                    LO                                                  )                            ⁢              t                                                                    =                                    α              ⁢                              xe2x80x83                            ⁢              cos              ⁢                              xe2x80x83                            ⁢                              (                                                      ω                    ⁢                                          xe2x80x83                                        ⁢                    LO                                    -                                      ω                    ⁢                                          xe2x80x83                                        ⁢                    RX                                                  )                            ⁢              t                        +                          β              ⁢                              xe2x80x83                            ⁢              cos              ⁢                              xe2x80x83                            ⁢                              (                                                      ω                    ⁢                                          xe2x80x83                                        ⁢                    im                                    -                                      ω                    ⁢                                          xe2x80x83                                        ⁢                    LO                                                  )                            ⁢              t                                          
And, these signals SII, SIQ are supplied to phase shifting circuits 13I, 13Q. The phase shifting circuits 13I, 13Q are configured with, for example, active filters using capacitors, resistors, and operational amplifiers. The phase shifting circuit 13I shifts the phase of the signal SII xcfx86 by and the phase shifting circuit 13Q shifts the phase of the signal SIQ by (xcfx86+90xc2x0). Thereby, the phase shifting circuits 13I, 13Q maintain the phase difference between the two inputted signals SII, SIQ within 90xc2x0xc2x11xc2x0 in a required intermediate frequency band.
Thus, the signal SIQ advances in phase by 90xc2x0 compared to the signal SII due to phase shifting circuits 13I, 13Q. The signals SII, SIQ are transformed into:       SII    =                            -          α                ⁢                  xe2x80x83                ⁢        sin        ⁢                  xe2x80x83                ⁢                  (                                    ω              ⁢                              xe2x80x83                            ⁢              LO                        -                          ω              ⁢                              xe2x80x83                            ⁢              RX                                )                ⁢        t            +              b        ⁢                  xe2x80x83                ⁢        sin        ⁢                  xe2x80x83                ⁢                  (                                    ω              ⁢                              xe2x80x83                            ⁢              im                        -                          ω              ⁢                              xe2x80x83                            ⁢              LO                                )                ⁢        t                                          SIQ          =                      xe2x80x83                    ⁢                                    α              ⁢                              xe2x80x83                            ⁢              cos              ⁢                              xe2x80x83                            ⁢                              {                                                                            (                                                                        ω                          ⁢                                                      xe2x80x83                                                    ⁢                          LO                                                -                                                  ω                          ⁢                                                      xe2x80x83                                                    ⁢                          RX                                                                    )                                        ⁢                    t                                    +                                      90                    ⁢                    xc2x0                                                  }                                      +                          β              ⁢                              xe2x80x83                            ⁢              cos              ⁢                              xe2x80x83                            ⁢                              {                                  (                                                            ω                      ⁢                                              xe2x80x83                                            ⁢                      im                                        -                                                                                                                                                                                xe2x80x83                                ⁢                                                      ω                    ⁢                                          xe2x80x83                                        ⁢                    LO                                    )                                            ⁢              t                        +                          90              ⁢              xc2x0                                }                                              =                      xe2x80x83                    ⁢                                                    -                αsin                            ⁢                              xe2x80x83                            ⁢                              (                                                      ω                    ⁢                                          xe2x80x83                                        ⁢                    LO                                    -                                      ω                    ⁢                                          xe2x80x83                                        ⁢                    RX                                                  )                            ⁢              t                        -                          βsin              ⁢                              xe2x80x83                            ⁢                              (                                                      ω                    ⁢                                          xe2x80x83                                        ⁢                    im                                    -                                      ω                    ⁢                                          xe2x80x83                                        ⁢                    LO                                                  )                            ⁢                              t                .                                                        
And, these signals SII, SIQ are supplied to an adding circuit 14, which produces an added signal:                     SIF        =                  xe2x80x83                ⁢                  SII          +          SIQ                                        =                  xe2x80x83                ⁢                                            -              α                        ⁢                          xe2x80x83                        ⁢            sin            ⁢                          xe2x80x83                        ⁢                          (                                                ω                  ⁢                                      xe2x80x83                                    ⁢                  LO                                -                                  ω                  ⁢                                      xe2x80x83                                    ⁢                  RX                                            )                        ⁢            t                    +                      β            ⁢                          xe2x80x83                        ⁢            sin            ⁢                          xe2x80x83                        ⁢                          (                                                ω                  ⁢                                      xe2x80x83                                    ⁢                  im                                -                                  ω                  ⁢                                      xe2x80x83                                    ⁢                  LO                                            )                        ⁢            t                    -                                                  xe2x80x83                ⁢                              α            ⁢                          xe2x80x83                        ⁢            sin            ⁢                          xe2x80x83                        ⁢                          (                                                ω                  ⁢                                      xe2x80x83                                    ⁢                  LO                                -                                  ω                  ⁢                                      xe2x80x83                                    ⁢                  RX                                            )                        ⁢            t                    -                      β            ⁢                          xe2x80x83                        ⁢            sin            ⁢                          xe2x80x83                        ⁢                          (                                                ω                  ⁢                                      xe2x80x83                                    ⁢                  im                                -                                  ω                  ⁢                                      xe2x80x83                                    ⁢                  LO                                            )                        ⁢            t                                                  =                  xe2x80x83                ⁢                              -            2                    ⁢          α          ⁢                      xe2x80x83                    ⁢          sin          ⁢                      xe2x80x83                    ⁢                      (                                          ω                ⁢                                  xe2x80x83                                ⁢                LO                            -                              ω                ⁢                                  xe2x80x83                                ⁢                RX                                      )                    ⁢                      t            .                              
Here, in the foregoing equation, since
xcfx89LOxe2x88x92xcfx89RX=2xcfx80(fLOxe2x88x92fRX) =2xcfx80fIF 
is given, the signal SIF is the objective intermediate frequency signal of the received signal SRX. And, even if the received signal SRX from the pre-selector 42 contains the image signal Sim, this intermediate frequency signal SIF does not contain the signal components generated by the image signal Sim because they are canceled.
Thus, the adding circuit 14 produces the intermediate frequency signal SIF [and the signal components of the angular frequency (xcfx89RX+xcfx89LO)] which are converted from the received signal SRX.
And, this intermediate frequency signal SIF is supplied to a band pass filter 15 for filtering the FM intermediate frequency components. This band pass filter 15 is configured with, for example, an active filter in the same manner as the phase shifting circuits 13I, 13Q, and the center frequency of the pass band is specified as 150 kHz. Thus, the band pass filter 15 removes undesired signal components such as the signal components of the angular frequency (xcfx89RX+xcfx89LO), and extracts only the intermediate frequency signal SIF.
And, this extracted signal SIF is supplied through a limiter 16 to an FM demodulating circuit 17 of the pulse count type to extract an audio signal. This audio signal is supplied through an audio frequency amplifier 18 to a speaker 44.
Thus, according to FIG. 1, since the FM intermediate frequency is set as low as 150 kHz, the intermediate frequency filter 15 can be integrated into a chip by configuring it with one of the active type. Accordingly, the part 10 surrounded by the chain line can be made into one chip IC.
Lowering the intermediate frequency as above will deteriorate the image characteristic; however, if the received signal SRX from the pre-selector 42 contains the image signal Sim, the signal components of the image signal can be canceled at the adding circuit 14 as mentioned above. Therefore, it is possible to listen to a broadcast without interference caused by the image signal Sim, that is, to improve the image characteristic.
Further, in the foregoing case, the IC 10 is applied to an FM receiver that employs a coil and a variable capacitor for the resonant circuit 43 connected to the IC 10. However, the IC 10 can be applied to an FM receiver of a synthesizer system that tunes stations.
FIG. 2 illustrates one example in which the IC 10 shown in FIG. 1 is applied to an FM receiver of the synthesizer system. In this case, the resonant circuit 43 for the local oscillation is configured with a local oscillation coil L43 and a variable capacity diode D43.
And, this resonant circuit 43 functions as a part of a PLL 430. That is, the local oscillation signal SLO obtained at the resonant circuit 43 is supplied through a buffer amplifier 431 to a variable frequency dividing circuit 432, where the frequency of the local oscillation signal SLO is divided into 1/N to be outputted as a signal S32. And, this frequency divided signal S32 is supplied to a phase comparator 433. On the other hand, an oscillation circuit 434 generates a signal of a reference frequency, for example 100 kHz, and this signal is also supplied to the phase comparator 433. The comparison output from this phase comparator 433 is supplied through a low-pass filter 435 and a buffer resistor R43 to the variable capacity diode D43 as a voltage to control the variable capacity diode D43.
Therefore, in the steady state, the frequency of the frequency divided signal S32 is equal to the oscillation frequency (=100 kHz) of the oscillation circuit 434, and the oscillation frequency 2fLO of the local oscillation signal SLO in this case is given by the following.
2fLO=Nxc3x97100[kHz]
And, here,
fRX=fLO+fIF 
fIF=150 kHz 
are given. Therefore, from these equations,
fRX=Nxc3x9750+150[kHz]
is obtained.
Therefore, to vary the frequency dividing ratio N of the variable frequency dividing circuit 432 by units of 2 within the range of 1517 to 1797 will vary the frequency fLO of the signals SLI, SLQ by steps of 100 kHz within the range of 75.85 MHz to 89.85 MHz, and the received signal frequency fRX can be varied by steps of 100 kHz within the range of 76 MHz to 90 MHz. Further, the frequency dividing ratio N can be set or varied by a microcomputer, in the same manner as a general PLL synthesizer receiver.
Thus, the IC10 can be used also for an FM receiver of the PLL synthesizer system.
Incidentally, in the foregoing IC 10, if the waveform of the oscillation signal SLO generated by the local oscillation circuit 21 is distorted as shown by a dotted line in FIG. 5, the length xcfx84+ of the positive half cycle does not coincide with the length xcfx84xe2x88x92 of the negative half cycle, that is, the duty ratio of the signal SLO deviates from 50%.
And, since the signals SLI, SLQ are designed to be inverted at the zero crossing points of the signal SLO, if the duty ratio of the signal SLO deviates 50%, the phase difference between the signal SLI and the signal SLQ deviates 90xc2x0 as shown by the dotted line in FIG. 5.
Accordingly, the phase difference between the image signal Sim contained in the signal SII and the image signal Sim contained in the signal SIQ, which are supplied to the adding circuit 14, deviates 180xc2x0. As a consequence, the image signal Sim contained in the signal SII and the image signal Sim contained in the signal SIQ will not be canceled sufficiently in the adding circuit 14, and the intermediate frequency signal SIF outputted from the adding circuit 14 will contain the residual components of the image signal Sim.
According to the experiments and theoretical calculations, if the deviation of the phase difference between the signal SLI and SLQ is within xc2x11xc2x0 from 90xc2x0 (namely, the phase difference is within 89xc2x0 to 91xc2x0), the image suppression ratio of 40 dB can be obtained. However, if the deviation of the phase difference becomes 2xc2x0, the image suppression ratio will go down to 34 dB.
Accordingly, it is considered to provide an automatic level control (ALC) circuit, to detect the level of the oscillation signal SLO generated by the local oscillation circuit 21, and to control the signal level at a specified value by the detected signal. That is, satisfying such an arrangement will reduce the distorted components (harmonics) contained in the oscillation signal SLO, and the waveform of the signal SLO becomes a pure sine wave to bring the duty ratio into 50%. Accordingly, the phase difference between the signal SLI and the signal SLQ becomes 90xc2x0, which gives the initially intended image characteristic.
However, if a receiver with an ALC circuit simply provided is used in an area where a strong interference radio wave such as a TV broadcasting wave near the TV transmission antenna is present, the strong interference signal will penetrate from the antenna 41 through the mixing circuits 12I, 12Q into the local oscillation circuit 21. And, when detecting a level of the oscillation signal SLO, the ALC circuit detects a level containing the interference wave signal. Consequently, the oscillation strength of the oscillation circuit 21 becomes lower than a desired value, and this will shut down the oscillation in the worst case, which is well conceivable.
The present invention intends to solve the foregoing problems.
In order to achieve the foregoing object, a receiver according to the invention contains a local oscillation circuit, a mixing circuit to perform a frequency conversion from a received signal into an intermediate frequency signal by using an oscillation signal generated by the oscillation circuit, an ALC circuit to detect a level of the oscillation signal and control the level of the oscillation signal at a specified constant value, and a control circuit to switch the operation of the ALC circuit from an operational state and a non-operational state and vice versa.
Therefore, in an area where the level of an interference wave is very high, the control circuit controls to switch the operation of the ALC circuit into the non-operational state, and thereby brings the oscillation circuit into a normal operation that generates the oscillation signal.